1. Field of the Invention
This invention is in the field of materials and more paticularly in the field of materials suitable as synthetic skin.
2. Description of the Prior Art
References to attempts to cover open wounds and severe burns go back to 1500 B.C. Breasted, J. H., Edwin Smith, Surgical Papyrus, vol. 1, Univ. of Chicago Press, Chicago, Illinois (1930), p. 83. Although a great deal of research effort has since been directed towards a functioning substitute for skin, no viable alternative for the autograft has yet been developed. However, the observations which have accumulated from the work already done in this area of research have resulted in the evolution of a number of basic concepts which may be applied to the design of a system for treating open wounds and third degree burns with a more realistic expectation of success.
Prior approaches which have been used to develop a skin substitute can be divided into four broad catagories, namely: homografts; modified dermal xenografts; synthetic polymeric structures; and, reconstituted collagen films.
The use of homografts in the treatment of massive burns is an accepted procedure at the present time. The source of the skin transplant may be a live donor or skin obtained from cadavers and preserved in a skin bank. The justification for the use of homografts is the necessity for reducing fluid loss, preventing infections, and reducing the area of scarring.
In the absence of immunosuppressive agents, however, homografts are almost invariably rejected. Rejection is apparently mediated primarily by the interception of graft vascularization which accompanies the onset of the immune reaction. Guthy, E. A., Billotte, J. B., Koumans, R. J. K., and Burke, J. F., in Research in Burns, Matter, P., Barclay, T. L. and Konickova, Z. (eds.), Hans Huber, Stuttgart, 1971. As is well known, the use of immunosuppressive agents to avoid rejection of such grafts is also accompanied by a host of problems.
Efforts to modulate the immunogenicity of homografts by organ culture techniques have been attempted by several investigators. After a number of conflicting reports, the results of a definitive investigation of such procedures was reported by Ninnemann and Good who concluded that modification of antigens in the cultured tissue had not been demonstrated in such attempts. Ninnemann, J. L. and Good, R. A., Transplantation, 18, 1 (1974).
An alternative approach to the use of homografts has been investigation of the possibility of modifying skin from animals. The basic goal of this approach is removal of those components in the dermis which elicit the production of host antibodies.
Oliver et al. pursued this approach by treating porcine dermis with trypsin to remove cellular and non-collagenous material. Oliver, R. F., Grant, R. A. and Kent, C. M., Brit. J. Exp. Path., 53, 540 (1972). This resulted in a graft material which was primarily insoluble collagen cast in the original morphology of the dermis, with a negligible level of antigenicity. The modified dermal collagen thus obtained was grafted on to full thicknss excised skin wounds in the pig and its fate compared to that of autografts and homografts of untreated dermis. The autografts behaved in the normal manner, described previously by Henshaw and Miller. Henshaw, J. R. and Miller, E. R., Arch. Surg., 658 (1965). The untreated homografts were daed by Day 5, with mononuclear cells present, and had begun to degenerate at the base by Day 10. By Day 20, the rejection of the homografts was substantially complete. With treated dermal collagen grafts, the lower part of the graft was repopulated with capillaries and fibroblasts by Day 5, while epidermal migration took place through the graft. Basophilic collagen lysis of the graft collagen started near Day 5 and was associated with infiltration of granulation tissue which progressively replaced collagen in the presence of multinuclear giant cells. By Day 20, the grafts were substantially replaced by granulation tissue and behaved like open wounds. This emphasizes the necessity for increasing the resistance of native collagen to lysis.
As a result of these experiments, Oliver et al. listed four requirements for a successful graft:
1. Dermal collagen fibers should persist unaltered for a long period, providing an essential structural framework for the reformation of the vascular and cellular elements of tissue;
2. The graft should not evoke foreign body reaction, which leads to eventual destruction of the newly cellularized graft;
3. The graft should provide a suitable dermal bed for the growth and development of normal epidermis; and,
4. The graft should suppress the formation of granulation tissue.
A third approach involves the use of synthetic polymeric structures. The literature is replete with references to the investigation of polymeric materials for a variety of biomedical applications including skin substitutes or temporary wound dressings. This is not surprising in view of the polymer scientist's capability of incorporating almost any set of physical and chemical (but, as yet, few biological) requirements into a polymeric structure. The investigations into the utility of polymeric films as skin replacements have, thus far, eliminated a considerable number of candidate materials but have resulted in useful insights into the requirements for a satisfactory skin replacement. For example, the use of a velours structure resulted in improved adhesion to tissue, and the development of methods of preparation of so-called biocompatible polymers with controlled pore size improved the possibility of synthesizing materials capable of inducing cellular migration and proliferation into the graft. See Hall, C. W., Liotta, D., Chidoni, J. J., Debakey, M. E., and Dressler, D. P., J. Biomed. Mat. Res., 1, 187 (1967); and Wilkes, G. L. and Samuels, S. L., J. Biomed. Mat. Res., 7, 541 (1973), respectively. Another promising approach involved polymerization of crosslinked polymers in the hydrogel form, thus providing added capability for encouraging cellular ingrowth and vascularization. Hubacek, J., Kliment, K., Dusek, J., and Hubacek, J. Biomed Mat. Res., 1, 387 (1967). The use of synthetic polymers in skin replacement has not so far led to solution of the problem, however, due mainly to the high incidence of infection and the inability of the materials evaluated up to now to encourage vascularization and epithelization.
Since the major constituent of normal skin is collagen, a logical approach to the development of a skin substitute would involve study of the fate of reconstituted collagen structures when placed in contact with living tissue.
This approach was used by a number of investigators using the general procedure of extracting the collagen from animals, purifying it to various degrees and converting it to films or other structures that were used as wound dressings or implanted in living tissue to determine their in vivo fate. Earlier work in this area demonstrated that collagen per se evokes a chronic inflammatory response with subsequent resorption of the implant. Pullinger, B. D. and Pirie, A., J. Path. Bact., 34, 341 (1942). Grillo and Gross were able to show that the rate of resorption of collagen could be reduced by controlled crosslinking with formaldehyde. They were also able to show that the immune response to reconstituted collagen implants was minimal. Grillo, H. C. and Gross, J., J. Surg. Res., 2, 69 (1962).
Enzymatically modified collagen has been prepared and evaluated by Rubin and Stenzel who showed that this treatment does not evoke as much cellular response as the untreated material. The explanation for this variation in behavior is that the enzyme used (proctase) effectively removes the telopeptides from the collagen molecule without destroying the native molecular structure. Rubin, A. L. and Stenzel, K. H., in Biomaterials (Stark, L. and Aggarwal, G., eds.), Plenum Press, N. Y. (1969). The use of reconstituted collagen sheets has not eliminated the problems of lysis, infection and prevention of tissue ingrowth and vascularization encountered by use of other approaches.